CMOS imaging devices were first developed between the early and late 1970s; however, because of their unacceptable performance they were generally overlooked until the early 1990s when advances in CMOS design yielded chips with smaller pixel sizes, reduced noise, more capable image processing algorithms, and larger imaging arrays. Among the major advantages of CMOS sensors are their low power consumption, master clock, and single-voltage power supply. This is in contrast with charge-coupled devices (CCDs) that often require five or more supply voltages at different clock speeds with significantly higher power consumption.
Both CMOS and CCD imaging devices sense light through the photoelectric effect, which occurs when photons interact with crystallized silicon to promote electrons from the valence band into the conduction band. Typically, a photodiode is used as the light sensing element of a pixel. A major advantage that CMOS image sensors enjoy over their CCD counterparts is the ability to integrate directly onto a sensor integrated circuit a number of processing and control functions that lie beyond the primary task of photon collection.
A pixel is the key element of a digital image sensor. The pixel includes various amplification devices, readout devices, and a light sensing device, such as a photodiode. When a broad wavelength band of visible light is incident on specially doped silicon semiconductor materials, a variable number of electrons are released in proportion to the photon flux density incident on the surface of the photodiode. Electrons are collected in a potential well until the integration (illumination) period is finished, and then they are either converted into a voltage or current (CMOS processors) or transferred to a metering register (CCD sensors).
Sensitivity is determined by a combination of the maximum charge that can be accumulated by the photodiode coupled to the conversion efficiency of incident photons to electrons and the ability of the device to accumulate the charge in a confined region without leakage or spillover. These factors are typically determined by the physical size and aperture of the pixel, and its spatial and electronic relationship to neighboring elements in the array. Pixels are typically organized in an orthogonal grid. The signals from all of the pixels composing each row and each column of the array must be accurately detected and measured (read out) in order to assemble an image from the photodiode charge accumulation data.
The most popular CMOS designs are built around active pixel sensor (APS) technology in which both the photodiode and readout amplifier are incorporated into each pixel. This technology enables the charge accumulated by the photodiode to be converted into an amplified voltage inside the pixel and then transferred in sequential rows and columns to the analog signal-processing portion of the chip. Thus, each pixel (or imaging element) contains, in addition to a photodiode, a number of transistors that converts accumulated electron charge into a measurable voltage, resets the photodiode, and transfers the voltage to a vertical column bus.
One of the important advantages of CMOS image sensors is that digital logic circuits, clock drivers, counters, and analog-to-digital converters can be placed on the same silicon substrate and at the same time as the pixel array. However, in order to guarantee low-noise devices with high performance, the standard CMOS fabrication process must often be modified to specifically accommodate image sensors. For example, standard CMOS techniques for creating transistor junctions in logic chips might produce high dark currents and low blue response when applied to an imaging device. Optimizing the process for image sensors often involves tradeoffs that render the fabrication scenario unreliable for common CMOS devices.